Atrophy in the gray matter of the medial prefrontal cortex in elderly patients was associated with reduced deep slow-wave activity (SWA) sleep and sleep-dependent episodic memory retention, according to results of a study published online in January in Nature Neuroscience.

The study was designed to compare the impact of sleep on short- and long-term memory skills in a group of healthy young adults (n=18; mean age, 20.4 years) with that of healthy older adults (n=15; mean age, 72.1 years). Long-term memory was assessed after 8 hours of sleep during which brain activity was monitored to track neuronal activity associated with transfer and storage of memory. High-quality sleep has long been known to be critical for memory consolidation.

Also known is that aging is independently associated with decreased volume of the prefrontal cortex, reduced SWA during non-rapid eye movement (NREM) sleep, and impaired capacity for memory. What this study adds is the correlation between natural brain atrophy and deep, SWA sleep-dependent memory.

All participants were asked to learn and retain sets of nonsense syllables. Short-term retention was tested after a practice period and the younger adults performed approximately 25% better than the older participants. EEG monitoring during the required 8 hours of overnight sleep revealed that on average, the rate of deep sleep among the older adults was 75% lower than that of those in the younger group. The following morning, the older adults’ memory of the practice words was 55% worse than that of the younger adults.

The research also found that estimated amount of prefrontal cortex atrophy in the older adults, as assessed by MRI before the study “statistically mediated the impairment of overnight, sleep-dependent memory.” An association was identified also between impaired memory performance and “persistent activation of the hippocampus and reduced task-related hippocampal-prefrontal cortex functional connectivity,” a deficit that may reflect poor memory transfer from short-term storage in the hippocampus to permanent memory in the prefrontal cortex.

Sleep quality, then, rather than the length of sleep appears to determine the brain’s ability to create permanent memory. The results also suggest that sleep disruption in the elderly, mediated by expected structural change in the brain, may be a factor that contributes to age-related cognitive decline in later life.

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